The Reports & Dashboards starting resources
|Salesforce Reporting Workbook||Click here||
This workbook is deprecated and has been for some time.
However, if you have no experience at all with reports and dashboards, it shows you the general outline of what is possible and has some nice examples.
Don't consider this as the end-all be-all but it can be a nice introduction.
|LEX Reports & Dashboard Trailheads||Click here||
The trailhead about reports & dashboards. The standard entry point if you haven't done it.
|Cross-Filters definition and video||Click here||
A quick intro video of how to have a report of companies with or without contacts using Cross Filters.
Is a simple solution to an oft-encountered pain point that some people solve with the Power of One.
|Power of One||Click here||Some people misuse it where a Cross Filter would work better, but it is still a staple of Salesforce reporting. PoO allows you to count unique occurrences of a record within a grouped record. Better explained in the link.|